Employee national insurance works differently than income tax. There is a single tax-free sum accessible per individual every tax year under income tax. There is a new limit for National Insurance for each employment as long as it is with a separate employer.
An employee, for example, has two jobs. It's the month of May 2021. Employees' weekly National Insurance 'Earnings Threshold' is £184. In general, weekly wages are £194. Earnings in the second job are £50 per week. In the first employment, national insurance is £1.20, or 12% of £194 minus £184. Because the wages in the second employment are less than £184 per week, no National Insurance Contributions are due.
The first job's employer will pay 'Secondary Contributions' of 13.8 percent above £170 per week. The second job's employer will not pay Employers' National Insurance.